In a gas turbine engine, there are many structures which can act as vibratory excitation sources, imparting unwanted vibration excitation to adjacent structures. Depending on the frequency of the resulting vibratory response and its amplitude, material fatigue and even failure can occur. This is particularly true for rotors that rotate within the engine.
Furthermore, some rotary mechanisms (rotors) are designed to direct a portion of the gas path to a secondary air system within the engine that is designed to prevent hot gas ingestion and to cool parts of the engine. If the air flow around the rotor is not adequately directed, insufficient pressurized air flow will be delivered to the secondary air system.